We studied the effect of one haemodialysis treatment on the plasma concentrations of intact parathyroid hormone (PTH), total immunoreactive PTH (determined with an antibody against the mid-molecular part of the hormone), and ionised calcium in 25 patients on maintenance dialysis for chronic renal failure. During dialysis the calcium concentration in the external fluid was 1.75 mmol/l, which led to an increase in the plasma ionised calcium concentration from 1.23 +/- 0.07 mmol/l (mean +/- SD) at commencement of dialysis to a final 1.35 +/- 0.07 mmol/l (P less than 0.001). The plasma concentration of intact PTH decreased from 27.4 +/- 26.3 pmol/l to 13.0 +/- 19.1 pmol/l (P less than 0.001) during the treatment. Total immunoreactive PTH did not change, reflecting poor dialysability of PTH and PTH fragments. We also compared the dialysis-induced changes in the plasma concentrations of intact PTH and ionised calcium with those induced by a calcium infusion test. We conclude that in the majority of dialysis patients, one haemodialysis treatment with a relatively high external fluid calcium concentration can be used to assess the suppressibility of secondary hyperparathyroidism.